It describes a couple of attacks. The first is that some platforms store their Secure Boot policy in a run time UEFI variable. UEFI variables are split into two broad categories - boot time and run time. Boot time variables can only be accessed while in boot services - the moment the bootloader or kernel calls ExitBootServices(), they're inaccessible. Some vendors chose to leave the variable containing firmware settings available during run time, presumably because it makes it easier to implement tools for modifying firmware settings at the OS level. Unfortunately, some vendors left bits of Secure Boot policy in this space. The naive approach would be to simply disable Secure Boot entirely, but that means that the OS would be able to detect that the system wasn't in a secure state[1]. A more subtle approach is to modify the policy, such that the firmware chooses not to verify the signatures on files stored on fixed media. Drop in a new bootloader and victory is ensured.

But that's not a beautiful approach. It depends on the firmware vendor having made that mistake. What if you could just rewrite arbitrary variables, even if they're only supposed to be accessible in boot services? Variables are all stored in flash, connected to the chipset's SPI controller. Allowing arbitrary access to that from the OS would make it straightforward to modify the variables, even if they're boot time-only. So, thankfully, the SPI controller has some control mechanisms. The first is that any attempt to enable the write-access bit will cause a System Management Interrupt, at which point the CPU should trap into System Management Mode and (if the write attempt isn't authorised) flip it back. The second is to disable access from the OS entirely - all writes have to take place in System Management Mode.

The MITRE results show that around 0.03% of modern machines enable the second option. That's unfortunate, but the first option should still be sufficient[2]. Except the first option requires on the SMI actually firing. And, conveniently, Intel's chipsets have a bit that allows you to disable all SMI sources[3], and then have another bit to disable further writes to the first bit. Except 40% of the machines MITRE tested didn't bother setting that lock bit. So you can just disable SMI generation, remove the write-protect bit on the SPI controller and then write to arbitrary variables, including the SecureBoot enable one.

This is, uh, obviously a problem. The good news is that this has been communicated to firmware and system vendors and it should be fixed in the future. The bad news is that a significant proportion of existing systems can probably have their Secure Boot implementation circumvented. This is pretty unsurprisingly - I suggested that the first few generations would be broken back in 2012. Security tends to be an iterative process, and changing a branch of the industry that's historically not had to care into one that forms the root of platform trust is a difficult process. As the MITRE paper says, UEFI Secure Boot will be a genuine improvement in security. It's just going to take us a little while to get to the point where the more obvious flaws have been worked out.

[1] Unless the malware was intelligent enough to hook GetVariable, detect a request for SecureBoot and then give a fake answer, but who would do that?[2] Impressively, basically everyone enables that.[3] Great for dealing with bugs caused by YOUR ENTIRE COMPUTER BEING INTERRUPTED BY ARBITRARY VENDOR CODE, except unfortunately it also probably disables chunks of thermal management and stops various other things from working as well.

We should be able to look at the experience of implementing many such systems such as on different kinds of media players such as game consoles, phones, set top boxes and see if there is any pattern. It seems that, like bitcoin, the difficulty increases over time because vulnerabilities get fixed and there isn't enough churn in the security critical parts to add new ones faster than they are patched.